This invention relates in general to a digital cinema and in particular relates to a digital projector using spatial light modulators to form an image on a display screen.
Conventional motion picture film projectors have proven successful in projecting quality images that satisfy a viewing audience. In recent decades, the overall cinematic experience has benefited by minor improvements in the quality of presentation. However, the most widely adopted technological improvement is cinema digital sound, rather than any change that improves the quality of the projected image. While the successful commercial development of some select large screen film formats, for example, the IMAX 70 mm format, and the supporting projection equipment has improved image quality, theatres equipped with this equipment are special venues, rather than outlets for traditional Hollywood films. Basically, since the introduction of the 1950""s of robust color films and xenon arc lamps, motion picture film projection technology has undergone minor improvements, with few technological breakthroughs.
Although the traditional 35 mm motion picture film system has deficiencies scratches such as dirt, and unsteadiness, which degrade the image quality, overall the system has set high standards for image quality. While the effective screen resolution for 35 mm film projection varies with print quality, 2000 line resolution is generally considered to be sufficient for electronic equivalent systems. Cinematic projection systems also provide a wide color gamut and a large frame sequential contrast ratio ( greater than 1,000:1). A large contrast range allows the system to properly render abrupt changes of lightness to darkness, such as may occur at dramatic scene changes. Furthermore, to meet the Society of Motion Picture and Television Engineers (SMPTE) projection standards of 16 ft. L (foot Lamberts) of center screen luminance, a typical cinema projector must provide 8,000 to 15,000 screen lumens, depending on the film format and screen size. Thus, the cinema experience demands very high levels of performance, particularly in comparison to the modern electronic business projector, which need only provide 1500 lumens and 250:1 contrast. Electronic or digital cinema projection systems must satisfy these basic cinematic system requirements, as well as meeting other requirements related to modulator and electronic artifacts, data compression, data security, and system robustness.
The earliest electronic projection system which could project xe2x80x9ccinema qualityxe2x80x9d images without the use of motion picture film was the Eidophor system, which was developed by E. F. Fischer (U.S. Pat. Nos. 2,391,451 and 2,605,352) in the 1940""s. The Eidophor used an electron beam to write images onto a reflective oil film. The oil film in turn was illuminated, and then imaged to the screen, through a Schlieren type optical system. An alternate system, called the xe2x80x9cTalaria,xe2x80x9d which was developed in the 1950""s by W. Glenn (U.S. Pat. Nos. 2,813,146 and 3,084,590) of General Electric, was similar to the Eidophor, except that it used transmissive, rather than reflective, oil films. Although both of these systems were successful in their own right, and were used successfully in cinematic projection demonstrations, neither had significant impact on the motion picture film projection industry.
Commercially available electronic projection systems are constrained by limited performance, particularly with respect to resolution, light efficiency, and contrast ratio. Typical systems, include those available from manufacturers such as JVC; Barco, headquartered in Ghent, Belgium; Christie Digital Systems, Inc., Kitchener, Ontario, Canada; and In Focus Corporation, Wilsonville, Oreg., among others. In general, these systems output between 500 and 3000 lumens and provide screen contrast ratios ranging from 100:1 to 400:1. These limitations constrain the use of such projection systems to home projection, business, concert, control room, and image simulation functions.
Recently there have been many proposals and technology demonstrations of alternate approaches to cinema. These approaches have ranged from proposed new film formats, to 3-D imaging or immersion systems, and to electronic display system. Most notably, Texas Instruments Inc. of Dallas, Tex., and Victor Company of Japan, Ltd. (JVC), or Yokohama, Japan, have publicly exhibited prototype electronic projection systems as candidates to replace 35 mm film in providing commercial quality cinema projection. While these prototype systems showed substantial merit, they have not yet matched or exceeded all the on-screen image quality and system flexibility standards set by the conventional film-based projection system. In particular, there are opportunities for improvement with respect to image resolution, pixelization, image contrast, color reproduction, and brightness needed to obtain the expected xe2x80x9clook and feelxe2x80x9d of film.
The most promising solutions for digital cinema projection employ, as image forming devices, one of two types of spatial light modulators, either a digital micro-mirror device (DMD) or a liquid-crystal device (LCD). Texas Instruments has demonstrated prototype projectors using one or more DMDs. DMD devices are described in a number of patents, for example, U.S. Pat. Nos. 4,441,791; 5,535,047; 5,600,383 (all to Hornbeck); and U.S. Pat. No. 5,719,695 (Heimbuch). Optical designs for projection apparatus employing DMDs are disclosed in U.S. Pat. Nos. 5,914,818 (Tejada et al.); 5,930,050 (Dewald); 6,008,951 (Anderson); and 6,089,717 (Iwai). While DMD-based projectors demonstrate some capability to provide the necessary light throughput, contrast ratio, and color gamut, inherent resolution limitations (current devices providing only 1024xc3x97768 pixels), high component and system costs have restricted DMD acceptability for high-quality digital cinema projection.
Alternatively, LCD devices appear to have advantages as spatial light modulators for high-quality digital cinema projection systems. Recently, JVC publicly demonstrated a LCD-based projector capable of high-resolution (providing 2,000xc3x971280 pixels), high frame sequential contrast in excess of 1000:1, and high light throughput (nominally, up to 12,000 lumens). This system utilized three vertically aligned LCDs (one per color) driven or addressed by cathode ray tubes (CRTs). While this system demonstrated the potential for an LCD based digital cinema projector, the system complexity, reliability, and cost are not well suited for commercial production. More recently, JVC has developed a new family of vertically aligned LCDs, which are addressed via a silicon backplane rather than by CRTs, although these new devices have not yet been used in digital cinema presentation. The JVC LCD devices are described, in part, in U.S. Pat. No. 5,570,213 (Ruiz et al.) and U.S. Pat. No. 5,620,755 (Smith, Jr. et al.). In contrast to early twisted nematic or cholesteric LCDs, vertically aligned LCDs potentially provide much higher modulation contrast ratios, in excess of 2,000:1. It is instructive to note that, in order to obtain on screen frame sequential contrast of 1,000:1 or better, the entire system must produce  greater than 1000:1 contrast, and both the LCDs and the polarization optics must each separately provide xcx9c2,000:1 contrast.
Obviously, the optical performance provided by LCD based electronic projection system is in large part defined by the qualities of the polarization optics and the LCDs. However, numerous other components, including the lamp source, light integration optics, the polarization converter, color filters and prisms, and waveplates also significantly impact performance. For example, as electronic projection systems modulate each red, green, and blue (R, G, B) color component separately, these systems also require color splitting and color recombination optics, including dichroic filters and color prisms, such as the familiar X-prism, commonly used for recombination. Thus, the relative success of an optical design for an electronic projection system is largely determined by the packaging and performance provided from the imaging sub-system, which includes modulator arrays, beam splitting optics, and a projection lens.
Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 (Shimomura et al.); U.S. Pat. No. 5,798,819 (Hattori et al.); U.S. Pat. No. 5,918,961 (Ueda); U.S. Pat. No. 6,010,121 (Maki et al.); and U.S. Pat. No. 6,062,694 (Oikawa et al.). Each of these example devices employ an arrangement of color splitting dichroic components to separate R, G, B light to their respective channels for modulation. One or more separate LCDs are then used for modulation within each channel. An X-prism then provides RGB color recombination. Polarization beamsplitters are used to selectively direct polarized light within each color channel. Performance is enhanced using low-stress beamsplitter designs, with specialized polarization beamsplitters used for each color channel. In the above patents, various solutions are disclosed to address a range of problems, including mounting methods to minimize thermally induced stress, symmetrical designs to minimize contrast and color shading, compact designs which limit the overall projector size, and the addition of optics for manipulating polarization states in order to maximize brightness, for example.
As is clearly shown in the above patents, polarization separation optics, such as polarization beamsplitters, are key components for determining the overall optical performance of electronic projection apparatus. In order to provide the level of contrast necessary for this application, a polarization beamsplitter must provide a high extinction ratio between modulated and un-modulated light. The polarization beamsplitter must also be able to meet the demands placed on the system for brightness. However, existing polarization beamsplitters have not been shown to provide the performance necessary for successful commercial use with high-quality digital cinema projects. In particular, typical polarization beamsplitters do not perform equally well at all wavelengths across the visible spectrum, resulting in undesirable color shading and contrast effects under some conditions. Likewise, conventional polarization beamsplitter devices also suffer from reduced performance versus incidence angle (that is, limited numerical aperture), which in turn effects the capability of attaining desired screen brightness levels.
The most common conventional polarization beamsplitter solution, which is used in many projection systems, is the traditional MacNeille prism, disclosed in U.S. Pat. No. 2,403,731. This device has been shown to provide a good extinction ratio (on the order of 300:1). However, this standard prism operates only over a limited range of angles (a few degrees). Because the MacNeille prism design provides good extinction ratio for one polarization state only, a design using this device must effectively discard half of the incoming light when this light is from an unpolarized white light source, such as from a xenon or metal halide arc lamp.
Conventional glass polarization beamsplitter designs, based on the MacNeille design, have other limitations beyond the limited angular response, which could impede its use for digital cinema projection. In particular, bonding techniques used in fabrication or thermal stress in operation, can cause stress birefringence, in turn degrading the polarization contrast performance of the beamsplitter. These effects, which are often unacceptable for mid-range electronic projection applications, are not tolerable for cinema projection applications. The thermal stress problem has recently been improved upon, with the use of a more suitable low photo-elasticity optical glass, disclosed in U.S. Pat. No. 5,969,861 (Ueda et al.), which was specially designed for use in polarization components. Unfortunately, high fabrication costs and uncertain availability limit the utility of this solution. Furthermore, while it would be feasible to custom melt low-stress glass prisms suited to each wavelength band in order to minimize stress birefringence, while somewhat expanding angular performance, such a solution is costly and error-prone. As a result of these problems, the conventional MacNeille based glass beamsplitter design, which is capable of the necessary performance for low to mid-range electronic projection systems, operating at 500-2,000 lumens with approximately 300:1 contrast, falls short of the more demanding requirements of full-scale commercial digital cinema projection.
Certainly, other polarization beamsplitter technologies have been proposed to meet the needs of an LCD based digital cinema projection system. For example, the beamsplitter disclosed in U.S. Pat. No. 5,912,962 (Li et al.), which comprises a plurality of thin film layers sandwiched between two dove prisms attempts to achieve high extinction ratios for both polarization states. Theoretically, the beamsplitter device disclosed in the Li et al. patent is capable of extinction ratios in excess of 2,000:1. When designed into a projection system with six LCDs (two per color), such a prism could boost system efficiency by allowing use of both polarizations for image projection. However, size constraints and difficulties in prism manufacture present obstacles to commercialization of a projection apparatus using this beamsplitter design.
As another conventional solution, some projector designs have employed liquid-immersion polarization beamsplitters. Liquid-filled beamsplitters have been shown to provide high extinction ratios needed for high-contrast applications and have some advantages under high-intensity light conditions. These devices, however, are costly to manufacture, and must be fabricated without dust or contained bubbles. Under conditions of steady use, they have exhibited a number of inherent disadvantages. Among the disadvantages of liquid-immersion polarization beamsplitters are variations in refractive index of the liquid due to temperature, including uneven index distribution due to convection. Leakage risk presents another potential disadvantage for these devices.
Wire grid polarizers have been in existence for a number of years, primarily used in radio-frequency applications and in optical applications using non-visible light sources. Use of wire grid polarizers with light in the visible spectrum has been limited, largely due to constraints of device performance or manufacture. For example, U.S. Pat. No. 5,383,053 (Hegg et al.) discloses use of a wire grid beamsplitter in a virtual image display apparatus where, notably, there is no requirement for high extinction ratio. In the Hegg et al. disclosure, an inexpensive wire grid beamsplitter is used as an efficient alternative, providing high light throughput when compared against conventional, glass-based beamsplitters. A second wire grid polarizer for the visible spectrum is disclosed in U.S. Pat. No. 5,748,368 (Tamada). While the device discussed in this patent provides polarization separation, the contrast ratio is inadequate for cinematic projection and the design is inherently limited to rather narrow wavelength bands.
Recently, as disclosed in U.S. Pat. No. 6,122,103 (Perkins et al.), higher quality wire grid polarizers and beamsplitters have been developed for broadband use in the visible spectrum. Among these are new devices commercially available from Moxtek Inc of Orem, Utah. While existing wire grid polarizers, including the devices described in U.S. Pat. No. 6,122,103, may not exhibit all of the necessary performance characteristics needed for obtaining the high contrast required for digital cinema projection, these devices do have a number of advantages. When compared against standard polarizers, wire grid polarization devices exhibit relatively high extinction ratios and high efficiency. Additionally, the contrast performance of these wire grid devices also has broader angular acceptance (NA or numerical aperture) and more robust thermal performance, with less opportunity for thermally induced stress birefringence than standard polarization devices. Furthermore, the wire grid polarizers are robust relative to harsh environment conditions, such as light intensity, temperature, and vibration. These devices perform well under conditions of different color channels, with the exception that response within the blue light channel may require additional compensation.
However, wire grid polarizers have not been satisfactorily proven to meet all of the demanding requirements imposed by digital cinema projection apparatus. Deficiencies in substrate flatness, in overall polarization performance, and in robustness at both room ambient and high load conditions have limited commercialization of wire grid polarization devices for cinematic projection. Furthermore, neither the wire grid polarizer, nor the wire grid polarization beamsplitter, provide the target polarization extinction ratio performance (nominally greater than 2,000:1) to achieve the desired projection system frame sequential contrast of 1,000:1 or better. Individually, both of these components provide less than 1,000:1 contrast under best conditions. Performance falls off further in the blue spectrum. Finally, the problems of designing an optimized configuration of polarization optics, including wire grid polarizers, in combination with the LCDs, color optics, and projection lens, have not been addressed either for electronic projection generally, or for digital cinema projection in particular. Thus, it can be seen that, while there are conventional approaches for digital cinema projection apparatus design, there is a need for an improved projection apparatus that uses the advantages of wire grid polarization beamsplitters to provide high-quality motion picture projection output.
It is an object of the present invention to provide a digital cinema projector for imaging of sequential color image frames onto a display surface.
Briefly, according to one aspect of the present invention, a digital cinema projector for projection of color images onto a display surface comprises a light source, which produces a beam of light. Beam-shaping optics homogenize and focus the beam of light and color splitting optics separate focus beam of light into separate color beams. A first modulation optics system comprises a prepolarizer, which prepolarizes a first color beam; a wire grid polarization beamsplitter, which transmits a first predetermined polarization state of the prepolarized beam; a reflective spatial light modulator, which alters the transmitted prepolarized beam with information and reflects the image bearing first color beam through the wire grid polarization beamsplitter, and a wire grid polarization analyzer, which transmits the image bearing first color beam and attenuates unwanted polarization components. A recombination prism combines the first color beam from the first modulation optical system with other image bearing color beams to create a full color image bearing beam. A projection lens system projects the full color image bearing beam onto the display surface.
It is a feature of the present invention that it employs a wire grid polarization beamsplitter within a digital cinema projector.
It is an advantage of the present invention that it provides a lightweight projection apparatus when contrasted against conventional digital cinema projector.
It is a further advantage of the present invention that it provides a robust solution for digital cinema projection, with reduced effects due to thermal stress when contrasted against conventional digital cinema projectors.
It is a further advantage of the present invention that it provides a high-performance digital cinema projector, with suitable light throughput, high contrast, and high resolution, with excellent color gamut performance.
It is a further advantage of the present invention that it offers the opportunity to fabricate a digital cinema projector that is lighter and more compact than existing equipment.
It is a further advantage of the present invention that it provides a digital cinema projection apparatus having minimum artifacts such as color shading, contrast shading, and flare light.
These and other objects, features, and advantages of the present invention will become more apparent to those skilled in the art upon reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.